The time to task failure for an isometric contraction with the elbow flexor muscles is twice as long when an individual sustains a submaximal force (force task) compared with maintaining a constant limb position while supporting an equivalent inertial load (position task). Although the net muscle torque generated by each subject is identical for the two tasks, indirect measurements indicate that central neural activity increases more rapidly during the position task. Furthermore, work completed in the initial cycle of this award has suggested that a difference in the time to task failure can exist without significant differences in inhibitory inputs, but that these inputs augment the difference in time to failure for the two tasks. Our model is that the position task involves reduced activation of the interneurons that are responsible for presynaptic inhibition of the feedback transmitted by group la afferents. Such suppression of presynaptic inhibition, which is modulated by both descending drive and feedback from peripheral afferents, will result in heightened excitation of the spinal motor neurons during the position task. We hypothesize that the position task is associated with greater net excitation from spinal and supraspinal sources, resulting in earlier recruitment of a finite pool of motor units and premature task failure. According to this hypothesis, the difference in the time to task failure is attributable to differences in the input received by the spinal motor neurons. We propose a research plan that will quantify the change in excitability of single motor units (Aim 1), compare the inputs that the spinal motor neurons receive from corticospinal neurons (Aim 2) and group la afferents of muscle spindles (Aims 3-4). and determine the level of activity in accessory muscles (Aim 5) during the two tasks. The experiments on the corticospinal neurons and the group la reflex pathway will involve evoked responses and will be performed in collaboration with Prof. Duchateau in Brussels. The outcomes will provide novel information on the physiological adjustments that occur during isometric contractions, which are the most common form of muscle activity, and will have direct application to the design of work tasks in ergonomics and the prescription of physical activities in rehabilitation.